The present invention relates to an electronic ballast for a high intensity discharge lamp.
Generally, there have been presented various types of arc tube lamps such as, for example, a fluorescent lamp commonly used in general houses, a lighting lamp used in factories, a street lamp installed on the street and so on. In this case, the fluorescent lamp, an ultraviolet rays lamp and a low pressure sodium vapor lamp are contained within low pressure discharge lamps, and the street lamp is included within high pressure discharge lamps. Referring to features of the low pressure discharge lamp, it has a long length of tube, so that the heat generated within the tube is discharged to the outside to thereby allow tube atmosphere to be same before or after the lamp is lighted. Therefore, the low pressure discharge lamp has a constant tube voltage at the time when the lighting of the lamp is initialized and after it is completed, so that it can easily adapt an electronic ballast without any problem. On the other hand, examples of the high pressure discharge lamp are a mercury lamp, a high pressure sodium vapor lamp, a metal halide lamp and the like, which are generally called a high intensity discharge(HID) lamp. Referring now to the features of the high intensity discharge lamp, it has a relative short length of tube but a high power. If the high intensity discharge lamp is lighted, the interior of the tube becomes at a substantially high temperature of plasma state, which causes air atmosphere within the tube to be tremendously high when compared with the time before the lamp is lighted. In the case where a constant amount of tube current is applied, the tube voltage upon the lighting of the lamp is much different from that after the lighting of the lamp is completed. In addition, even when the lamp is stabilized and operated, under the effect of a negative resistance impedance in the lamp, an impedance is high over the tube voltage and contrarily, it is low below the tube voltage. To light such the high intensity discharge lamp, a prior art leakage transformer type ballast has been generally employed. The leakage transformer functions as a current source and has a function of limiting an amount of the current regardless of the impedance of lamp. As a result, when the lamp is initially lighted the tube voltage approaches a potential of zero voltage, and when the lighting of lamp is stabilized, it is changed to a normal potential. To high the inductance of the leakage transformer, however, a long coil for winding the transformer is required, which renders the efficiency of the leakage transformer really deteriorated.
To solve the disadvantage of the leakage transformer, an electronic ballast using an electronic control method is newly employed. Owing to the introduction of the electronic control method, there are generated several advantages in that the efficiency thereof is high, the volume thereof is small, the weight thereof is light, and an added external control is easily executed.
A direct current(DC) power source applied to the conventional electronic ballast should be a regulated voltage source and, if a ripple voltage exists in the regulated voltage source, the light of lamp is flickered or is automatically turned off due to the negative resistance impedance characteristic of the lamp. Furthermore, the power source outputted to the lamp should be a current source. To this end, the regulated DC power source should be inverted into the lamp power. Therefore, the electronic ballast circuit is preferably comprised of a converter part and an inverter part. Various kinds of control methods in the converter part and the inverter part are suggested, and hereinafter, an explanation of the advantages and disadvantages thereon will be in detail discussed.
Converter Part
1) condenser input type control method: An alternating current(AC) line power of about 50 Hz or 60 Hz is full wave rectified, and to suppress a ripple factor in the full wave rectified power, a large capacity of condenser is installed. In the control method, however, an input power factor falls to a percentage of about 55%. Moreover, upon application of the initial power, an inrush current greatly flows. Accordingly, the control method is not adapted for a large capacity of electronic device, because of the above-mentioned problems.
2) choke input type control method: To solve the problem occurring in the condenser input type control method, a choke coil is inserted into the front end of the condenser. A ripple rate is variable in accordance with the current value, and to decrease the ripple rate, a large capacity of choke coil is necessary. However, the choke input type control method is not well adapted because the choke coil for improving the power factor should be designed in a large size.
3) chopper regulator type control method: This method is used to decrease the capacity of the choke coil by using a switching transistor. The method is classified into a step-down converter method and a step-up converter method. In the step-down converter method, a control voltage is lower than an input power, and contrarily, in the step-up converter method, the control voltage is higher than the input power. Accordingly, the method can arbitrarily adjust the control voltage by using both the step-down converter and the step-up converter. In the method, however, there occur the problems in that the power loss can not be avoided because of the installation of the switching transistor and noises are greatly generated due to a reverse recovery current of a flywheel diode, which is combined with the switching transistor.
Inverter Part
1)DC current source control method: In the high intensity discharge lamp, a mercury lamp contains mercury within the tube thereof, and does not form any material therein. A high pressure sodium vapor lamp or a metal halide lamp spreads a lighting color material over the electrode thereof or pastes a part of the tube with the material. If a DC current source is applied to the lamp, the two poles of the lamp have different temperatures from each other. In this case, there occurs no problem in lighting the mercury lamp, but there occurs the problem in that the sodium vapor lamp or the metal halide lamp can not emit a desired lighting color. As a result, for the application of the lamp power source, the regulated DC voltage source should be inverted into the alternating current source. Therefore, this method can be adapted only for the embodiment of the mercury lamp.
2) low frequency inverter type control method in a full bridge manner: A full bridge switching transistor inverts a frequency into a low frequency and controls an output in a chopper regulator type. And, the full bridge switching transistor generates a current source in the step-down circuit on the chopper regulator. If the full bridge switching transistor does not operate as the current source, theoretically the output current infinitely flows due to the negative resistance impedance characteristic of the lamp. The above method has the advantages of a good input power factor and a stabilized power control, but has the disadvantages of a low efficiency of about 84%, lots of noises and a large number of parts.
3) a high frequency inverter type control method: If the power of inverter part is the regulated DC power source, the method employs a series resonant inverter to thereby connect a series resonant condenser in parallel with the lamp. In the series resonant circuit, lighting of the lamp can be easily implemented since a series resonant inductance is operated as a current source. The power control of lamp is executed with a higher frequency than a resonant frequency. In a frequency adjustable control, the current or power of lamp is detected to thereby control the frequency of switching transistor. The above method has the advantage of a simple output circuit. On the other hand, there occurs a defect in that since the DC power source should be the regulated power source, the AC line power has to be applied to pass through the regulated voltage circuit. In addition, if the lamp is automatically turned off due to an unexpected accident during the lighting, the impedance of the series resonant circuit is greatly low to cause a great large amount of current to flow to the switching transistor, which results in the damage on the switching transistor.
An object of the present invention is to provide an electronic ballast for a high intensity discharge lamp which is capable of reducing the number of parts to thereby maintain a high power factor and prevent power loss generated in the circuit thereof.
To achieve this and other objects according to the present invention, there is provided an electronic ballast for a high intensity discharge lamp including: a converter part comprised of first to fourth rectifying diodes, each of the rectifying diodes which converts an input current into a DC current on which a ripple is loaded, and a choke coil and a first condenser for increasing a conduction angle of the input current; an inverter part comprised of first and second switching transistors to which a rectified DC current source is applied, a second condenser for supplying a charged voltage to a DIAC to turn on the DIAC, to thereby switch the second switching transistor, such that the conduction of the second switching transistor enables a free resonant signal to be excited to an output matching part, and first and second gate wave shaping integrated circuits to which the resonant signal detected by a gate transformer is applied, the first and second gate wave shaping ICs applying the applied signal to a gate of the first switching transistor to pass a controlled voltage in a gate protection Zener diode through a voltage source and a first current source, applying a positive gate voltage signal to gates thereof when a reflection signal voltage is increased and if the voltage is decreased, turning on a first transistor as potentials on the gates are higher than those on the gate transformer, to thereby decrease the gate potential to a negative gate potential to finally reach the potential of the voltage source, such that since the first and second switching transistors are synchronous with the reflection signal and are thus switched, the outputs therefrom are resonant and continually oscillated; the output matching part comprised of a resonant coil and first to third resonant condensers for calculating a time constant, and a transformer for establishing a winding rate to be matched with a tube voltage, to thereby return a reflection power generated when an output is not in a resonant state to an absorbed DC power source part; the absorbed DC power source part comprised of first and second diodes for inputting the reflection power generated from the output matching part, third and fourth condensers to which the inputted force is charged, to thereby reduce a ripple of a DC voltage, and third and fourth diodes for compensating the charged voltage to the third and fourth condensers through the first and second diodes in the state where the DC voltage is at a low level; a power control part comprised of a first operational amplifier for controlling a detected value from a first DC current detection resistor and second and third DC voltage detection resistors as a multiplied value in an output control integrated circuit and for controlling a second current source to control the current of the gate transformer, a first resistor on which the controlled current is outputted as a voltage, a second operational amplifier for operating the voltage with a reference voltage, a second resistor to which the operated value is outputted, a third resistor on which the outputted value is set as a power set value to input the power set to the first operational amplifier, a first comparator to which a detected value of a temperature sensor is inputted, a fourth resistor for setting a comparison value and for cutting an output of the first operational amplifier if the detected temperature is higher than the set value in the first comparator, a second comparator to which a detected value of a photo sensor is inputted, a fifth resistor for setting a comparison value and for cutting the output of the first operational amplifier if the detected external illuminance is higher than the set value in the second comparator, a first switch for removing the output cut of the first operational amplifier, a third comparator for comparing a value appearing at the time when the current value controlled in the second current source is changed to the voltage with a reference voltage and if a large amount of the current flows, for cutting the output of the first operational amplifier, a timer for cutting the output of the first operational amplifier if the lamp is not lighted within a desired relighting time period, a sixth resistor and a fifth condenser for determining a time constant for setting a time, and an undervoltage lockout for cutting the output of the first operational amplifier if a power source voltage is in short supply; and the gate transformer for detecting the resonant signal in the output matching part to thereby apply the detected signal to the first and second gate wave shaping integrated circuits.